Uroporphyrin I, which accumulates in body tissues of congenital erythropoietic porphyria patients, can undergo an enzymatic one-electron reduction to the porphyrin anion radical when a suitable reducing cofactor is present. We have demonstrated that anaerobic microsomal incubations containing NADPH and uroporphyrin I give an electron spin resonance spectrum of a porphyrin anion free radical . Inhibitor studies indicate that NADPH-cytochrome P-450 reductase is the electron donor. This radical undergoes a second-order decay due to nonenzymatic disproportionation of the radical. Aerobic microsomal incubations were also investigated for the reduction of oxygen to superoxide by monitoring oxygen consumption and the spin-trapping of superoxide. These experiments demonstrated that electron transfer from the porphyrin radical to molecular oxygen does occur, but due to the slow formation of the radical anion, no oxygen consumption above the basal level could be detected in the microsomal incubations. The photoreduction of uroporphyrin I in aerobic and anaerobic incubations was also investigated. Similar results have been obtained with photofrin II, a photo-activated antitumor agent. The oxidation of a variety of porphyrins to cation free radicals by peroxidases also has been investigated. Since the enzyme intermediate of horseradish peroxidase, compound I, is itself a porphyrin IX cation radical, this work will have implications for electron transfer as well as porphyrin metabolism.